Stable, Low Cost Microcomputer Digital-To-Analog Instrument Control*

Two methods of computer-controlling an electronic instrument are described. One method employs a computer having as few as two digital-to-analog converters. A second method is shown which can be used if a 16line digital output is available. Features of either method include low cost, high accuracy and stability, low noise and ease of installation. (Submitted to Review of Scientific Instruments) * Work supported by the Department of Energy, contract DE A CO3 76s F005 15. A common way of implementing computer control of an electronic instruments is to purchase an “off the shelf” digital-to-analog conversion (DAC) unit, install it in the computer and connect it to the proper control point of the instrument circuit via a shielded cable (Figure 1). Potential problems with this scheme include 1) finding a reasonably priced DAC card for the computer being used, 2) obtaining sufficient resolution and stability from the unit chosen, 3) minimizing noise pickup on the control line connecting the computer and the instrument, and 4) curing ground loop problems which are likely to arise. In this Note, we show that a high level of performance can be achieved at low cost using a computer equipped with as few as two DACs and a user-constructed instrument control board containing common components. The primary devices needed for the instrument control board are low voltage optoisolators, common counter chips, a 16-bit DAC chip and, optionally, a high performance operational amplifier. If the host computer has a M-line parallel output available the implementation is simplified and will be discussed below. Referring to the block diagram (Figure 2), output lines from three computer DACs, which may be of low resolution (< 12 bits), are connected, through optisolators, to four 4-bit counters which are cascaded to form a 16bit counter. One DAC is used to toggle the clock input of the first 4-bit counter. The second DAC is used as a count reset line. The third DAC sets the direction of counting. If the computer contains only two DACs, bi-directional operation is not possible and the counters are hard wired to set the counting direction. The output states of the counters achieved when they are reset is determined by hard wiring the data input pins on each counter chip, usually to all zeros or ones. The opto-isolators perform a very important function. They provide complete electrical isolation between the computer and the instrument. The ground _ connections on the light-emitting diode side of the opto-insulators are entirely separate from all ground and power supply connections on the photo-transistor side of the isolators and the rest of the circuit. Ground loops between computer